POSITION-EQUAL

(Position item seq) is the least index (zero-based) of the
element item in the string or list seq, if in fact item is
an element of seq. Otherwise (position item seq) is nil.

(Position-equal item lst) has a guard of (true-listp lst).
Position-equal has the same functionality as the Common Lisp
function position, except that it uses the equal function to
test whether item is the same as each successive element of
lst. See position and see position-eq.